[issue38881] unexpected behaviour of random.choices with zero weights

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

Either raising, or treating a zero-weight-sum as undefined behaviour and documenting that the sum of the weights should be positive sounds fine to me.

-1 on the suggestion to (deliberately, by documented design) choose at random in this case. Mathematically, this situation doesn't make sense: as Tim said, it's analogous to choosing from an empty population.

Ex: you have `nred` red balls and `nblue` blue balls in a bag. If you want to simulate drawing a single ball from the bag, then

   random.choices(["red", "blue"], [nred, nblue])

does the job. But in the case where `nred = nblue = 0`, the bag is empty and it's not possible to draw anything; in that case I'd expect an error. I definitely wouldn't expect to get either a red ball or a blue ball (with equal probability).

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38881>
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